It is proposed that regulatory mutants controlling the phosphoribosyl transferase structural gene locus in mammalian cells be isolated and characterized. This will be accomplished through the use of a selective system in which overproducing revertants will be selected from a mutant strain which possesses low levels of the enzyme. Temperature sensitive variants for the HPRT gene will be isolated, and that clone which is the least leaky will be utilized for subsequent studies. A large number of revertants will be isolated at the restrictive temperature in the reverse selective HAT medium, and these will be screened preliminarily for their temperature sensitivity. Clones which still retain their temperature sensitive status, but which produce such large quantities of the enzyme that they are able to grow in HAT medium will be assumed to be regulatory mutants which overproduce the defective gene product. Such mutants will be characterized genetically through cell hybridization and biochemically by purification and characterization of the mutant enzyme. The object of these studies will be to establish the type of regulatory locus (linkage relationship to the HPRT gene, dominance/recessivity status) and the properties of the enzyme synthesized in the mutants and their revertants. It is asserted that such studies will lay a basis for a future series of investigations on the molecular basis of the regulatory locus.